This invention relates to tuning of optical fiber components, and in particular provides a tuning element as well as an optical component provided with such a tuning element.
Various optical fiber components are known for use within optical fiber communication systems, and in particular for overcoming various distortions which arise as an optical signal is transmitted along a fiber span. For example, the use of Bragg reflection gratings is known for causing wavelength-dependent delays, which compensate for dispersion effects.
Many optical materials exhibit different responses to optical signals of different wavelengths. Chromatic dispersion, often simply referred to as xe2x80x9cdispersionxe2x80x9d, is one well-known resulting phenomenon, in which the index of the refraction of a medium is dependent on the wavelength of an optical wave. Dispersion can cause optical waves of different wavelengths to travel at different speeds in a given medium, since the speed of light is dependent on the index of refraction. The dispersion of optical materials in general relates nonlinearly to the wavelength.
In many applications, an optical signal is composed of spectral components of different wavelengths. For example, a single-frequency optical carrier may be modulated in order to impose information on the carrier. Such modulation generates modulation sidebands at different frequencies from the carrier frequency. Also, optical pulses, which are widely used in optical data processing and communication applications, contain spectral components in a certain spectral range. The dispersion effect may cause adverse effects on the signal due to the different delays on the different spectral components.
Dispersion in particular presents obstacles to increasing system data rates and transmission distances without signal repeaters in either single-channel or wavelength-division-multiplexed (xe2x80x9cWDMxe2x80x9d) fiber communication systems. Data transmission rates of tens of Gbit/s may be needed in order to meet the increasing demand in the marketplace. Dispersion can be accumulated over distance to induce pulse broadening or spread. Two adjacent pulses in a pulse train thus may overlap with each other at a high data rate due to dispersion. Such pulse overlapping can often cause errors in data transmission.
There have been various proposals for overcoming the dispersion effect, including use of a Bragg grating. Such gratings are known both with linearly chirped (i.e. varying) grating periods and with non-linearly chirped grating periods, in order to achieve the desired spectral response along the length of the optical carrier (fiber or waveguide).
A spectral component in an optical signal having the Bragg wavelength is reflected back from a Bragg grating. Other spectral components are transmitted through the grating. The Bragg wavelengths at different positions in the fiber grating are differentiated by the chirping of the grating period, so that the Bragg wavelength of the fiber grating changes with the position. As the grating period increases or decreases along a direction in the fiber grating, the Bragg wavelength increases or decreases accordingly. Therefore, different spectral components in an optical signal are reflected back at different locations and have different delays. Such wavelength-dependent delays can be used to negate the accumulated dispersion in a fiber or waveguide link.
To use a chirped Bragg grating, an optical circulator is typically used to couple the input optical signal to the grating and to route the reflected signal. An optional optical isolator or anti-reflection termination may be placed at the other end of the grating to reject any optical feedback signal.
To provide a tuneable fiber grating, it is necessary to alter the chirp of the grating. Known methods for providing tuning of Bragg gratings within optical components rely upon mechanical, thermal or thermo-mechanical arrangements. These techniques have relatively slow response times, high power dissipation and may also suffer from reliability problems.
According to the invention, there is provided a tuning element for tuning an optical component by adjusting the length of an optical fiber used within the optical component, the tuning element comprising;
an elongate piezoelectric element, the length of the piezoelectric element being controllable by applied control signals; and
a surround around the piezoelectric element and contacting the ends of the piezoelectric element, wherein the surround has a width, transverse to the longitudinal axis of the piezoelectric element, which varies when the length of the piezoelectric element is varied thereby amplifying changes in length of the piezoelectric element,
wherein opposite sides of the surround are provided with an optical fiber fixing portion, and the piezoelectric element is provided with an opening for receiving a fiber, such that a portion of a fiber can pass transversely through the opening, the length of the portion being adjustable by controlling the length of the piezoelectric element.
The arrangement of the invention uses piezoelectric actuators, which have a fast response time and provide reliable operation. The use of a surround around a piezoelectric element, so that changes in length of the element are converted into changes in width of the surround, results in amplification of the change in dimension of the piezoelectric element.
Each optical fiber fixing portion may comprise a tube through which the fiber can pass, with one end of the tube contacting the surround and the other end of the tube being connected to the fiber. This enables the length of the fiber being stretched by the tuning element to be different to the width of the surround. Thus, the tuning element can be designed simply to provide the required change in length of the fiber, and the optical fiber fixing portion enable this change in length to be provided to the desired length of fiber.
The tuning element may comprise two elongate piezoelectric elements arranged side by side and parallel to each other, both surrounded by the surround. In this case, the separation between the two piezoelectric elements will be selected in dependence upon the length of the fiber to be stretched by the tuning element.
Each optical fiber fixing portion may be provided for fixing at least two optical fibers. Thus, in an optical component which uses a number of fibers, for example a number of Bragg gratings, which are to be tuned in the same manner, this can be achieved with a single tuning element.
The two optical fiber fixing portions may additionally be arranged such that the length of the two fibers are different, so that different tuning is provided for the two different fibers.
The use of two piezoelectric elements also enables independent control of the tuning of two fibers using the single tuning element. To achieve this, one fiber is connected to one side of the surround and to one of the piezoelectric elements, and the other fiber is connected to the other side of the surround and the other piezoelectric element. Each piezoelectric element results in a change in shape of one side of the surround, and this has an effect only on one of the optical fibers in the tuning element.
To enable easy assembly of the tuning element, the optical fiber fixing portion may comprise a ferrule for attachment to the optical fiber, the ferrules having a narrow portion for engagement in an opening in the surround, and wider portions on each side of the narrow portion which prevent movement of the ferrule through the opening. The ferrule can then be attached to the surround by sliding the narrow portion into a slot in the surround. In addition, the opening in the piezoelectric element may also comprise a slot, so that the optical fiber, with ferrules attached, may simply be slotted into the tuning element during assembly.
The invention also provides a tuneable optical component comprising an optical fiber and a tuning element of the invention.
As described above, the fiber may be fixed to opposite sides of the surround, or alternatively the fiber may be fixed in one position to the surround, and in another position to the piezoelectric element, particularly in the case where two piezoelectric elements are provided and the tuning element is for tuning two fibers simultaneously and independently.
Typically, a Bragg grating is written into the fiber, and the tuning operation then results in a change in the chirp of the Bragg grating. This Bragg grating may form part of a dispersion compensator or a tuneable fiber filter. The tuned fiber may also contain a laser cavity defined by mirror or grating reflectors.
The invention also provides a method of tuning an optical component using the tuning element of the invention.